weapon enhancements
Because the legendary weapons were not made to be class specific. Frostfang 100% fits necros (cold/chill is totally in tune with us from a lore perspective), but yes the rest don’t really fit well.
Also, your scepter idea involves doing something completely unrelated to skins, you’re asking for a new skill set, which isn’t going to happen on scepter at this point.
You can always skin a weapon with fine transmutation crystals and there are weapons with dark looks as well as skins on the TP or available in non-gold currencies.